1. Field of the Invention
The invention generally relates to a process for the production of a diesel fuel. More particularly, the invention is directed to a process for increasing the amount of medium oil which can be used as a diesel fuel. Thus, the total oil yield is increased by about 4 to 6 percent compared with previously obtained results.
2. Description of the Prior Art
The present invention is directed to an improvement to a process for the production of diesel fuel in which the conventional process includes the steps in which a medium-grade oil obtained from coal is derived as a first runnings for the subsequent treatment in a refining and hydrocracking stage for the extraction of diesel fuel. The conventional process includes the hydrogenation of the coal in the presence of grinding oil which oil is obtained from the process, which also yields hydrogen-containing circuit gas and a finely divided catalyst or sump phase. The unliquefied solids are separated from the sump phase in a hot precipitator at about the same tempertures and pressures as in the liquefaction reactor, in which hydrogenation takes place. The vapor-form head product of the hot precipitator is condensed in an intermediate and a cold precipitator which takes place with the simultaneous recovery of the circulation gas. The aforedescribed conventional process produces a light oil which boils below 200.degree. C. and a medium oil as a first batch runnings, which boils between 200.degree. and 325.degree. C., as well as a grinding oil.
The above-outlined conventional process has associated therewith several drawbacks. Particularly, only 65 to 70 percent of the entire oil yield from the coal consists of the medium oil which is used to produce diesel fuel. As a result of the further processing of the medium oil to produce a diesel fuel, there is produced appreciable fractions, namely 60 to 65 percent of the products which boil below 185.degree. C. and which are no longer suitable for use in diesel engines.
Some examples of the refining processes are: U.S. Pat. No. 4,447,312, which relates to a process for preparing diesel fuel from coal-derived light fuel oils; U.S. Pat. No. 4,409,092, which describes a combination process for upgrading hydrocarbon fractions obtained from oil products of coal processing; and U.S. Pat. No. 4,318,797, in which the invention provides a process and an apparatus for hydrogenative liquefaction of coal to produce high yields of gasoline fraction, and optional yields of diesel and residue fraction, all of superior quality. All of the afore-mentioned patents are incorporated herein by reference. U.S. Pat. No. 4,251,346, also incorporated herein by reference, describes a process for coal liquefaction. The comminuted coal is slurried in a solvent or pasting oil and digested, normally under hydrogen pressure and catalytic conditions. The solvent or pasting oil is obtained wholly or mostly by recycling from the distilled fractionation of the reaction products. Additionally, U.S. Pat. No. 4,018,663, also incorporated herein by reference, relates to an improved coal liquefaction process which enables conversion of a coal-oil slurry to a synthetic crude refinable to produce larger yields of gasoline and diesel oil. The process is characterized by a two-step operation applied to the slurry prior to catalytic desulfurization and hydrogenation, in which the slurry undergoes partial hydrogenation to crack and hydrogenate asphaltenes, and the partially hydrogenated slurry is filtered to remove minerals prior to subsequent catalytic hydrogenation.